Lactose is a disaccharide (12-carbon) sugar that occurs in mammalian milk. It consists of a molecule of glucose and one of galactose joined by a glycosidic bond and is broken down into these simple sugars by the action of the enzyme lactase which is secreted in the small intestine. Human milk contains about 6.7% lactose; cows' milk about 4.5% lactose. It exists in milk at body temperature mainly in two tautomeric forms (α and β) in approximately the proportion 2 to 3.
People with lactase deficiency have a reduced ability to digest lactose – a condition known as lactose intolerance. In rare cases, lactose intolerance may occur in someone who is not deficient in lactase.
Lactose is important in cheese-making, when lactic bacteria turn it into lactic acid, so souring the milk in the production of cheese curd.